Nickel tin bonding system with barrier layer for semiconductor wafers and devices

ABSTRACT

A light emitting diode structure is disclosed that includes a light emitting active portion formed of epitaxial layers and carrier substrate supporting the active portion. A bonding metal system that predominates in nickel and tin joins the active portion to the carrier substrate. At least one titanium adhesion layer is between the active portion and the carrier substrate and a platinum barrier layer is between the nickel-tin bonding system and the titanium adhesion layer. The platinum layer has a thickness sufficient to substantially prevent tin in the nickel tin bonding system from migrating into or through the titanium adhesion layer.

RELATED APPLICATIONS

The present application is a divisional application of Ser. No.11/844,127 filed Aug. 23, 2007 now U.S. Pat. No. 7,910,945 for, “NickelTin Bonding System with Barrier Layer for Semiconductor Wafers andDevices,” which is a continuation in part of Ser. No. 11/428,158 filedJun. 30, 2006 for, “Nickel Tin Bonding System for Semiconductor Wafersand Devices.” The disclosures of both of these applications areincorporated entirely herein by reference.

BACKGROUND

The present invention relates to the structure and composition of metalbonding systems used to attach substrate wafers carrying light emittingdiodes (LEDs) to other substrate wafers during LED manufacture.

Light emitting diodes (LEDs) are a class of photonic semiconductordevices that convert an applied voltage into light by encouragingelectron-hole recombination events in an appropriate semiconductormaterial. In turn, some or all of the energy released in therecombination event produces a photon.

A typical LED includes p-type and n-type epitaxial layers (“epilayers”)that form a p-n junction through which current injection occurs toproduce the recombination events. These epilayers are typically grown ona substrate of the same or a different semiconductor. Epilayers can beproduced with relatively high crystal quality and thus enhance thequality and operation of the resulting devices. The substrate portion ofthe device may not require the same level of quality, or in some cases,substrates formed of the same material as one or more of the epilayersare not readily available (or available at all).

Because of their wide bandgap and direct transition characteristics, theGroup III nitride materials are favored for shorter wavelength lightemitting diodes; i.e., those that emit in the blue, violet, andultraviolet portions of the electromagnetic spectrum. The Group IIInitride materials can, either in conjunction with diodes of other colorsor with phosphors, produce white light. At the same time, Group IIInitride substrate crystals of an appropriate size and quality aredifficult or impossible to obtain. As a result, LEDs based on the GroupIII nitride material system typically include Group III nitrideepilayers on sapphire or silicon carbide (SiC) substrates.

For a number of reasons, when the epitaxial layers of light-emittingsemiconductor materials are formed (typically by chemical vapordeposition (“CVD”) growth) on a substrate, the resulting precursorstructure can be in some cases added to an additional substrate. Thesecond substrate may be other than a semiconductor or if it is asemiconductor, it is not necessarily present for semiconductingpurposes. For example, in commonly assigned and co-pending U.S. PatentApplication Publication Number 20060060877, a second substrate is usedfor mounting and fabrication purposes and to form a portion of a finalLED structure. No. 20060060877 is incorporated entirely herein byreference. As set forth therein and elsewhere, the manufacture ofcertain types of LEDs includes one or more steps to reduce the thicknessof the original substrate (e.g., because the original substrate isthicker in order to make the initial manufacturing steps easier).Related background is set forth in commonly assigned U.S. PatentApplication Publications Nos. 20060049411, 20060060872, 20060060874, and20060060879, and the contents of each of these is likewise incorporatedentirely herein by reference.

In other structures, light emitting diodes are mounted to secondsubstrates in order to reverse (flip) their normal orientation. Stateddifferently, in a typical orientation, the substrate is mounted to alead frame and the epitaxial layers form the emitting face of the LED.In a flip chip orientation, however, the epitaxial layers are mountedtowards the lead frame and the substrate provides the light emittingsurface of the LED. Various steps in the process of manufacturing suchflip chip diodes can require that the LED-carrying substrate wafer bejoined to another substrate wafer either temporarily or permanently. Insome flip-chip embodiments, the LED-carrying substrate wafer is removedfrom the epitaxial layers after the epitaxial layers are mounted to thetemporary or permanent substrate wafer.

The conventional manner of joining the LED-carrying substrate wafer(also referred to herein as the “growth” wafer or substrate) to anothersubstrate wafer (the “carrier” wafer or substrate) includes the use ofvarious metal layers in a manner either identical or analogous tosoldering or brazing when permanent bonding is desired. In manycircumstances, a layer of titanium (Ti) is formed or deposited onto therespective surfaces to be joined, and then additional layers of bondingmetals are added to form a bonding metal structure on each of the firstand second substrates (sometimes referred to as the donor and acceptorsubstrates).

For numerous reasons, gold (Au) has historically been a predominantelement in these bonding metal layers. Because it resists oxidation andother chemical reactions (which makes it, of course, historicallyvaluable for jewelry and related items), gold also is attractive for itscorrosion resistance; i.e., avoiding undesired reaction with itssurroundings. Gold's ability to form relatively low melting point alloysor compounds (with respect to pure gold) also makes it ideal forsoldering purposes.

Nevertheless, the expense of gold, even in small amounts used inindividual semiconductor devices, becomes significant when multipliedover the millions of individual light emitting diodes that the marketnow demands.

As another factor, soldering wafers to one another requires someapplication of heat. Thus, a soldering step used to join an LEDsubstrate wafer to a second substrate wafer will heat the LEDs to someextent. As is well understood by those of ordinary skill in this art,raising the temperature of the light emitting semiconductor epitaxiallayers raises the corresponding probability of generating defects in theepitaxial layers. Typically, gold-tin based soldering (bonding, brazing)systems require temperatures above about 300° C. Although epitaxiallayers of, for example, Group III nitride materials, can theoreticallywithstand such temperatures, in reality these temperatures significantlyincrease the probability that the bonding step will generate noticeabledefects.

As yet an additional factor, when individual LEDs are separated from awafer and mounted on a lead frame (e.g., to form a lamp), they aretypically mounted on the lead frame with another soldering step. If theLED already contains a solder bond, the existing solder bond shoulddesirably remain unaffected by the temperatures required to solder thewafer-bonded chip to the lead frame. Thus, the temperature at which theLED can be soldered to the lead frame will be limited by the meltingtemperature that the substrate-substrate bond. Stated differently, thethermal characteristics of the substrate-substrate bonding metallurgymay unfavorably limit the type of solder that can be used to join anindividual LED to an individual lead frame.

Accordingly, a need exists for taking advantage of the improvedcharacteristics of nickel-tin bonding systems while avoiding theproblems raised when tin migrates through the nickel layer to formundesired species such as free tin, titanium tin alloys, or otherthermally unstable intermetallic compounds.

SUMMARY

In one aspect the invention is a light emitting diode structure thatincludes a light emitting active portion formed of epitaxial layers anda carrier substrate for supporting the active portion. A bonding metalsystem that predominates in nickel and tin joins the active portion tothe carrier substrate. At least one titanium adhesion layer is betweenthe active portion and the carrier substrate, and a platinum barrierlayer is between the nickel tin bonding system and the titanium adhesionlayer. The platinum layer has a thickness sufficient to substantiallyprevent tin in the nickel tin bonding system from migrating into orthrough the titanium adhesion layer.

In another aspect, the invention is a light emitting diode precursorstructure that includes a light emitting active portion formed of atleast two epitaxial layers of Group III nitride and a carrier substratefor supporting the active portion. A bonding metal structure is betweenthe active portion and the carrier substrate. The bonding metalstructure includes a middle layer of tin between two outer layers ofnickel with the relative amount of tin being greater than the amountwould be consumed by reacting with either nickel layer alone, but lessthan the amount that would provide a functional reaction excess of tinover both nickel layers. A titanium adhesion layer is between the activeportion and the bonding metal structure and a platinum barrier layer isbetween the titanium adhesion layer and the bonding metal structure forpreventing tin in the bonding structure from migrating into or thoroughthe titanium adhesion layer.

In yet another aspect, the invention is a precursor structure for alight emitting diode that includes a growth structure and a carrierstructure for being joined together. The growth structure includes agrowth substrate, light emitting epitaxial layers on the growthsubstrate, and a metal bonding system on the epitaxial layers forjoining to the carrier structure. The growth structure metal bondingsystem is formed predominately of a layer of nickel and a layer of tin,with an adhesion layer of titanium between the nickel layer and theepitaxial layers and a platinum barrier layer between the titaniumadhesion layer and the nickel layer. The carrier structure includes acarrier substrate, a titanium adhesion layer on the carrier substrate, aplatinum barrier layer on the titanium layer, and a nickel layer on theplatinum layer for joining to the growth structure. When the bondingmetal system on the growth structure and the nickel layer on the carrierstructure are joined and heated, the respective platinum barrier layersprevent tin from migrating into or through either of the titaniumadhesion layers.

In another aspect, the invention is a light emitting diode structure ona lead frame. In this aspect, the invention includes a light emittedemitting active portion formed of epitaxial layers, a carrier substratefor supporting the active portion, and a lead frame or equivalentstructure attached to the carrier substrate with a solder composition. Abonding metal system joins the active portion to the carrier substratein the barrier layer is between the bonding metal system and theepitaxial layers. The barrier layer is formed of the material and has athickness sufficient to substantially preclude the formation ormigration of free metals or alloys that have melting points lower thanthe melting point of the solder composition.

The foregoing and other objects and advantages of the invention and themanner in which the same are accomplished will become clearer based onthe followed detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional schematic view of an LED structure with anickel-tin bonding system.

FIGS. 2-5 are cross-sectional photographs of structures with nickel-tinbonding systems.

FIGS. 6, 7 and 8 are Auger spectra of the surface illustrated in FIG. 5taken after successive bombardments with argon.

FIGS. 9 and 10 are photographs of LED structures with nickel-tin bondingsystems.

FIGS. 11-12 are EDS spectra from two positions taken from the surfaceillustrated in the photograph of FIG. 10.

FIG. 13 is a cross-sectional photograph of a structure according to thepresent invention.

FIG. 14 is a cross-sectional schematic view of a structure according tothe present invention.

FIG. 15 is a cross-sectional schematic view of another embodiment of astructure according to the present invention.

FIG. 16 is a cross-sectional schematic view of another embodiment of thepresent invention.

DETAILED DESCRIPTION

The present invention is an improved metal bonding system that isparticularly useful in certain structures of light emitting diodes.

The nature and operation of individual LED devices are well understoodin the art and will not be repeated in detail herein. Appropriatereferences include Sze, PHYSICS OF SEMICONDUCTOR DEVICES, 2d Edition(1981); Schubert, LIGHT-EMITTING DIODES, Cambridge University Press(2003) and, Zetterling, PROCESS TECHNOLOGY FOR SILICON CARBIDE DEVICES,Electronic Materials Information Service (2002)

The bonding system set forth in the '158 application successfullyaddresses a number of the issues raised when gold predominates byforming a bonding system predominantly of nickel (Ni) and tin (Sn). Asset forth therein, although the melting point of tin is relatively low(232° C.) alloys formed of nickel-tin compounds that include betweenabout 30 and 70 weight percent tin have melting points that are allabove 750° C. Thus, the bonding system described in the '158 applicationsuccessfully meets the criteria of withstanding thermal excursions inthe 250-300° C. range that take place during fabrication steps that arecarried out after wafers are bonded to one another.

Devices formed using the nickel-tin bonding system in the '158application have achieved many of the desired goals. Nevertheless, astheir use has increased it has been discovered that tin unexpectedlymigrates through the adjacent nickel layers. This migration appears totake place even when the proportional amounts of tin and nickel are suchthat all of the tin would be expected to be present in the form of thenickel-tin alloy. Several resulting problems have been observed. First,when the migrating tin reaches the titanium adhesion layer that istypically present (e.g., Paragraph 0047 of the '158 application) it canform titanium-tin compounds that are thermally unstable at thetemperatures required for the later fabrication steps. Such compoundscan and will react in the solid state during later fabrication steps andas they undergo thermal aging as the end device is used. Theseintermetallic compounds can also cause delamination problems evenwithout actually melting. Second, the tin can migrate and form portionsof free tin adjacent the titanium adhesion layers. Because of tin's 232°C. melting point, such portions of free tin or thermally unstable alloyscreate positions in the structure that have a higher propensity forseparating at the fabrication temperature steps.

Theoretically, increasing the amount of nickel in the layer adjacent atin layer should successfully preclude any given amount of tin frommigrating through the nickel layer. In practice, however, tin continuesto migrate even when the thickness of the adjacent nickel layers isdoubled. This indicates that the tin has a reasonably high mobility innickel and that thickness alone may not solve the problem. Moreimportantly, merely adding thickness to a bonding layer (and thus toLEDs, lamps and packages that incorporate the bonding layer) offers noparticular structural or electronic advantage. Although a nickel layercould probably be formed that absolutely precluded tin from migrating,its relative thickness would raise corresponding disadvantages such asfabrication difficulty, size, and cost.

In one sense, the invention can be understood as an improvement in thestructures described in the co-pending and previously incorporated '158application. Accordingly, FIG. 1 is representative of such a lightemitting diode structure broadly designated at 20. The diode 20 includesrespective epitaxial layers 21 and 22 typically formed of galliumnitride or another appropriate Group III Nitride. The epitaxial layersare supported by a carrier substrate 23 through a system of barriermetals 24 a pair of titanium adhesion layers 25 and 26 and the nickeltin bonding system 27 described in the '158 application. In exemplaryembodiments, the carrier substrate is selected from the group consistingof aluminum, copper silicon and silicon carbide, with silicon beingprevalent in a number of examples.

It will be understood that the description and figures herein areexemplary rather than limiting of the diode structures that can benefitfrom the present invention. Thus, although LEDs can have structures asbasic as one p-type and one n-type layer of GaN, LEDs can also be formedfrom multiple epitaxial layers, some of which may include InGaN as wellas buffer layers that often include AlGaN. The light emitting structurescan include p-n junctions, quantum wells, multiple quantum wells andsuperlattice structures.

It will also be understood that structural terms such as, “on,” “above,”and “between,” are used herein in a broad sense to indicate relativepositions are relationships rather than (for example) immediate contactbetween the recited items. In any given portion of the description, themeaning will be clear in context.

FIG. 2 is a cross-sectional micrograph of a light emitting diode havingthe basic structure schematically illustrated in FIG. 1. The commonelements are commonly numbered with FIG. 1. Accordingly, the galliumnitride epitaxial layers are indicated by the brackets 21, 22, thebarrier metals at 23, the titanium layers at 25 and 26, the nickel-tinbonding system at 27 and the silicon substrate at 23. FIG. 2 alsoillustrates, however, a void area 30, certain portions of which appearas a dark shadow immediately under the barrier layers 23 and otherportions of which appear as the light gray surface above the titaniumlayer 25. As set forth earlier, the void space results from thedelamination that occurs when tin migrates and reacts with the titaniumlayers 25 and 26, or forms free tin, or forms other undesired thermallyunstable intermetallic compounds.

FIG. 3 is another view of the same structure as FIG. 2 but with aslightly different orientation that emphasizes the void 30 created bythe delamination.

FIG. 4 is an enlargement of portions of FIG. 3 and illustrates theresidual titanium layer 25 just below the barrier metal layers 24. Thevoid space created by the delamination is again visible as the darkspace 30.

FIGS. 5 through 8 confirm the nature of the problem solved by thepresent invention. FIG. 5 is yet another view of a structure accordingto the '158 application, but in an orientation turned approximately (butnot exactly) 90 degrees from FIGS. 2, 3 and 4. An Auger spectroscopicanalysis was carried out on the portion of the delaminated surfacelabeled as “2.” In FIG. 5, a delaminated portion of the epitaxial layers21, 22 appear in the lower left-hand corner, the top of the nickel tinbonding metal system 27 corresponds to the target designated “2” and thesilicon substrate is again designated at 23.

The Auger spectrum of FIG. 6 was taken prior to any sputter bombardmentwith argon. It indicates the presence of titanium at the kinetic energyvalues of 380 and 416 electron volts (eV).

FIG. 7 shows the spectrum from the same target area after 30 seconds ofargon sputtering at a reference sputter rate in gold of about 5angstroms per second. The large peak at approximately 510 eV indicatesthe presence of oxygen from the native oxide that forms on the layerwhen the delaminated portion is exposed to the atmosphere. In comparisonto FIG. 6, peaks representing tin are becoming evident at approximately423 and 435 eV, but peaks that would be expected for nickel (for exampleat about 700 eV) are completely absent. This spectrum demonstrates thattin and titanium are present at the surface (rather than nickel andtin), which in turn indicates that the nickel is neither holding the tinas an alloy nor preventing tin from migrating to or through the titaniumlayer.

FIG. 8 is the same spectrum taken after 90 seconds of bombardment and inwhich peaks characteristic of tin at 426 and 432 eV are evident. FIG. 8again illustrates the absence of nickel, thus confirming the results ofFIG. 7 and the existence of the problem.

FIGS. 9 through 12 illustrate a different aspect of the same problem,namely the presence of significant amounts of free tin. Whereappropriate, similar portions of the structure carry similar numerals asin the earlier figures. FIG. 9 is a cross sectional view of a structurefrom which the epitaxial layers have been removed in order to analyzethe bonding structure. Thus, the metal barrier layers are againdesignated at 24 and the nickel-tin bonding system at 27. The voidcreated by delamination is again labeled at 30 and the silicon carriersubstrate at 23. FIG. 9 also illustrates, however, a plurality ofportions of free tin 31. FIG. 10 is a plan view of the delaminatedsurface of the bonding layer 27.

FIGS. 11 and 12 are EDS spectra that confirm that the portionsdesignated at 31 are tin. FIG. 11 is an EDS spectra taken of the lumpobject designated “1” in FIG. 10. As FIG. 11 indicates, the sample isalmost entirely tin without the presence of titanium or nickel.

When the same analysis is taken of the location labeled “3” (i.e., otherthan the lump structures), the spectra indicates the presence of gold,tin, and titanium, but again little or no nickel.

FIG. 13 is a cross-sectional micrograph of an LED structure according tothe present invention broadly designated at 34 that includes theplatinum barrier layer. Where appropriate, like elements carry the samereference numerals as in the earlier figures. Accordingly, the structure34 includes two gallium nitride epitaxial layers 21, 22, although in thephotographs they appear as a single layer. The barrier metal layers areagain designated at 24, and the nickel tin bonding system (containing asmall amount of gold) is again designated at 27. The titanium adhesionlayers appear as the dark lines 25 and 26 on opposite sides of thenickel tin bonding system 27. The platinum barrier layer appears as thelighter line 35 above the nickel tin bonding system 27.

In preferred embodiments and as set forth in the '158 application, theamounts of nickel and tin are selected to make sure that all of the tinwill react with nickel. In the most preferred embodiments, the amountsof nickel tin and any other elements present (e.g., gold) are such thata complete reaction takes place leaving behind little or no unreactedmetal that can undergo diffusion, thermal aging, or any otherdisadvantageous processes over time.

Platinum represents an advantageous barrier metal because platinum canform intermetallic phases with any tin that reaches the platinum-tininterface. In this regard, it has been discovered that the barrier layeris most effective when it reacts with the tin. Thus, an effectivebarrier layer reacts with tin rather than merely blocking it fromdiffusing. These platinum-tin phases are stable within the temperatureranges at which the diodes undergo further fabrication and tend toremain stable during thermal aging. Stated differently, the presence ofthe platinum layer prevents formation of low melting point compounds,other thermally unstable intermetallic compounds, or tin migration tothe adhesion interface opposite the nickel interface.

Those familiar with the fabrication of light emitting diodes willrecognize that the characteristics of the barrier layers and relatedstructures described herein are relevant in the context of the normaland expected length of time during which the diode will be exposed toany given temperature. In general, diodes of the type described hereinwill typically be exposed to steps such as chemical vapor deposition (toadd passivation layers, etc.) at temperatures in the neighborhood of200° C. for a time on the order of one hour. Various annealing processes(for example to obtain or improve ohmic contacts) take place at somewhatmore elevated temperatures (e.g., about 290° C.) for periods of about anhour. Attaching a diode to a lead frame (“die attach”) using a reflowprocess typically takes place at a higher temperature (315°-350° C.) fora shorter period of time; e.g., 5 or 10 seconds. Typical operatingconditions are less than 50° C. High temperature (100° C.) operatinglifetime has been demonstrated in excess of 1000 hours with nodegradation of the bond.

Thus, it will be understood that the barrier layers and metal systemsdescribed herein maintain their desired properties for at least the timerequired to carry out these fabrication processes and typically muchlonger.

As the relatively clean appearance of the center portion of FIG. 13indicates, the platinum barrier layer 35 prevents tin in the nickel-tinbonding system 27 from migrating to or through the titanium layer 25.Accordingly, FIG. 13 is exemplary of a structure that avoids the type ofdelamination visible in (for example) FIGS. 1 and 9.

Although titanium adhesion layers are frequently present on both thecarrier side and the epitaxial side of the bonding metal system, theinvention offers its advantages even in the absence of titanium; i.e.,in structures where other adhesion layers known to those skilled in theart are used, or where platinum can act as the adhesion layer, or wherean adhesion layer is not included.

FIG. 13 also illustrates the problem solved by the platinum barrierlayer 35. In particular, in the portion of the photograph of FIG. 13that is labeled with the arrow at 36, the titanium adhesion layer 26tends to disappear into or otherwise be obscured by the nickel-tinbonding system 27. This is further evidence that, in the absence of theplatinum barrier layer, the tin in the nickel tin bonding system 27 willmigrate either into or through (or both) the titanium layer 26 and formlow melting point titanium-tin compounds, other thermally unstableintermetallic compounds, or free tin.

FIG. 13 also includes an area labeled at 38 that illustrates the onsetof localized interaction of the platinum layer 35 at temperatures ofabout 330° C. The titanium layer 25 remains well-defined above the area38. When the structure of FIG. 13 is heated further (e.g., to about 350°C.) more such localized interaction has been observed.

In exemplary embodiments, the localized interactions between platinumand tin (e.g., region 38 in FIG. 13) tend to become more significant atbonding temperatures approaching about 330° C. and under the amount offorce being applied (typically on the order of about 6500 Newtons). Atlower temperatures, such as about 240° degrees centigrade, the localizedinteractions between platinum and tin are not evident. Accordingly, apractical bonding temperature range starts at about the melting point oftin (232° C.) up to about 350° degrees centigrade, with a preferredrange being between about 240° and 330° C.

The barrier layer offers structural advantages on the epitaxial side ofthe metal bonding system, or on the carrier substrate side, or both.Thus, the invention includes all three possibilities. As a generalobservation to date (rather than a limitation of the invention) diodeswithout the barrier tend to delaminate on the epitaxial side more thanon the carrier substrate side.

FIG. 13 also illustrates that the invention has advantages in thecontext of the thickness of the bond line because the barrier layer makea thinner bond line more practical. The term “bond line” is generallywell-understood in the art and as used herein refers to the metalbonding system 27. In the invention, the bond line is typically lessthan 6 microns (μm) thick with exemplary embodiments being less than 3μm thick. The thin bond line offers several advantages such as reducedfabrication costs and reduced stress in the metal system. In comparison,high stress can create a relatively high degree of wafer bowingparticularly when employed in a thin (e.g., less than 150 μm) wafer.Such bowing can increase the difficulty of later fabrication andprocessing. Wafer bowing can also cause processing difficulty in thickerwafers (e.g., about 600 μm) during steps such as maintaining a vacuum ona chuck or photolithography. A number of different factors can, ofcourse, influence wafer stress, (e.g., bonding temperature, bondingpressure, deposition technique), but the invention provides theopportunity to minimize the bond line thickness and thus minimize anystress caused by excess bond line thickness.

As noted earlier, the platinum barrier layer (35 in FIG. 13) precludesthe movement of tin beyond the barrier layer 35. This enables the tin tobe constrained by the platinum barrier layer 35 rather than by simplyincreasing the amount of nickel present in the bonding layer 27 (andthus avoiding increasing the bond line thickness).

FIG. 14 is a schematic diagram of a light emitting diode according tothe invention broadly designated at 37 that includes the elementsillustrated in FIG. 13, but also illustrating a second platinum barrierlayer on the carrier wafer side of the device. The gallium nitridelayers are again indicated at 21 and 22, the metal barrier layers at 24,and the substrate at 23. The titanium adhesion layers are illustrated at25 and 26 on opposite sides of the nickel-tin bonding system 27.

FIG. 14 includes the platinum barrier layer 35 illustrated in FIG. 13,and also includes a second platinum barrier layer 40 on the carriersubstrate side of the device 37.

In exemplary embodiments of the invention the bonding metal system 27 isa metal alloy that is more than 75 percent nickel and tin and in somecases more than 85 percent nickel and tin. If present, gold is limited,with exemplary embodiments containing less than 50 percent by weightgold and more typically less than 20 percent by weight gold.

FIG. 15 illustrates the invention in the context of a precursor growthstructure broadly designated at 42 and a carrier structure broadlydesignated at 43. The growth structure 42 includes a growth substrate 44which in exemplary embodiments is formed of silicon carbide because itslattice match with the Group III nitrides is better than that of othersubstrate materials such as sapphire. The growth structure 42 includesGroup III nitride epitaxial layers 45 and 46 which in exemplaryembodiments are formed of gallium nitride, but as noted earlier caninclude InGaN and can also include more complex structures such asheterojunctions, double heterojunctions, quantum wells, multiple quantumwells, and superlattice structures.

The barrier metals typically present are designated at 48.

The growth structure 42 includes a metal bonding system on the epitaxiallayers 45 and 46 for joining the growth structure 42 to the carrierstructure 43. The growth structure metal bonding system is formedpredominantly of a layer of nickel 47 and a layer of tin 50 with anadhesion layer of titanium 51 between the nickel layer 50 and theepitaxial layers 45, 46 and a platinum barrier layer 52 between thetitanium adhesion layer 51 and the nickel layer 47.

The carrier structure 43 includes a carrier substrate 53 typicallyselected from the group consisting of silicon and silicon carbide, asecond titanium adhesion layer 54 on the carrier substrate 53, a secondplatinum barrier layer 55 on the second titanium layer 54 and a secondnickel layer 56 on the second platinum layer 54 for joining the carrierstructure 43 to the growth structure 42.

When the bonding metal system on the growth structure 42 and the nickellayer on the carrier structure 43 are joined and heated, the respectiveplatinum barrier layers 52 and 54 prevent tin from the layer 50 frommigrating into or through either of the titanium adhesion layers 51 or54.

FIG. 15 also illustrates that in this embodiment the growth structure 42and the carrier structure 43 can each include a flash layer of gold forimproving the bond when the structures are joined. The growth structure42 has a flash layer of gold 57 on the tin layer 50 and the carrierstructure 43 has a flash layer of gold 60 on the second nickel layer 56.

In exemplary embodiments, the thickness of the tin layer 50 in thegrowth structure 42 is between about five and 10 times the thickness ofthe nickel layer 47 in the growth structure 42.

In exemplary embodiments, which are illustrative rather than limiting ofthe claimed invention, the layers on the growth structure 42 have thefollowing thicknesses: the titanium adhesion layer 51 is approximately100 nanometers, the platinum barrier layer 52 is approximately 150nanometers, the nickel layer 47 is approximately 200 nanometers, the tinlayer 50 is 2 microns thick, and the gold flash layer 57 is 30nanometers.

On the corresponding carrier structure 43 the titanium adhesion layer 54is 100 nanometers, the platinum barrier layer 54 is 1500 angstroms, thenickel layer 56 is 300 nanometers, and the gold flash layer 60 is 5nanometers.

In another embodiment (not shown) an additional layer of about 100nanometers of gold is positioned on the growth structure 42 between the2 micron layer of tin and the 200 nanometer layer of nickel.

In other embodiments, the amount of nickel in either the growthstructure 42 or the carrier structure 43 can be increased to 300 or 400nanometers depending upon the desired final structure and composition.

As set forth in the '158 application, the respective structures 42 and43 illustrated in FIG. 15 are heated when placed together under moderatepressure to form the bonded structure. Thereafter the growth substrate44 can be thinned or removed depending upon design choice.

FIG. 16 illustrates the invention in the context of a lead frame. As incertain of the other drawings, previously-referenced elements carry thesame reference numerals as they did in the other drawings.

Accordingly, FIG. 16 illustrates a light emitting diode structurebroadly designated at 60. The structure 60 includes a light emittingportion formed of the epitaxial layers 21 and 22. A carrier substrate 23supports the epitaxial layers 21 and 22. The carrier substrate isattached to a lead frame 61 with a solder composition 62. It will beunderstood that the lead frame and solder composition are illustratedschematically and in representative fashion, and that light emittingdiodes are attached in the same or analogous manners to other fixturesand that such other analogous and equivalent structures fall within thescope of the invention.

A bonding metal system 27 joins the epitaxial layers 21 and 22 to thecarrier substrate 23. A barrier layer 63 is positioned between thebonding metal system 27 and epitaxial layers 21, 22. The barrier layer63 is formed of a material and has a thickness sufficient tosubstantially preclude the formation or migration of free metals oralloys that have melting points lower than the melting point of thesolder composition 62, or that demonstrate thermal instability at suchtemperatures.

As set forth with respect to the previous embodiments, when the metalbonding system 27 predominates in nickel and tin, platinum serves as anexemplary barrier layer. As also set forth with respect to the otherembodiments, the diode structure 60 typically also includes an adhesionlayer 25 between the metal bonding system 27 and the epitaxial layers21, 22 and with the platinum barrier layer 63 being positioned betweenthe adhesion layer 25 and the nickel-tin bonding system 27. As in theother exemplary embodiments, the adhesion layer includes titaniumbecause of its favorable properties for this purpose.

FIG. 16 also illustrates the barrier metal system 24 that isconventionally (but not necessarily) used in the structure along with asecond titanium adhesion layer 26 immediately adjacent the carriersubstrate 23 and an additional platinum barrier layer 64 between thebonding metal system 27 and the second adhesion layer 26 for likewisepreventing free metals or alloys with low melting points (tin inparticular) or other thermally unstable intermetallic compounds fromforming or migrating to or through the titanium adhesion layer 26 at thecarrier substrate. As noted earlier, however, if the barrier layer alsoprovides satisfactory adhesion properties, the specific titaniumadhesion layer may be omitted.

In the drawings and specification there has been set forth a preferredembodiment of the invention, and although specific terms have beenemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being defined inthe claims.

1. A light emitting diode comprising: a light emitting active portionformed of at least two Group III based epitaxial layers; a bonding metalstructure that predominates in nickel and tin on said active portion;and a platinum barrier layer between said active portion and saidbonding metal structure for substantially preventing tin in said bondingmetal structure from migrating through said barrier layer, wherein saidbonding metal structure includes gold in an amount greater than 0.3percent and less than 36 percent by weight of said bonding metalstructure.
 2. A light emitting diode according to claim 1 wherein saidGroup III based nitride epitaxial layers comprise gallium nitride.
 3. Alight emitting diode according to claim 1 wherein the thickness of saidtin layer is between about five and 10 times the thickness of either ofsaid nickel layers.
 4. A light emitting diode according to claim 1,comprising a titanium adhesion layer between said barrier layer and saidactive portion.
 5. A light emitting diode according to claim 1 furthercomprising a carrier substrate that is configured to support the activeportion, wherein the carrier substrate is selected from the groupconsisting of aluminum, copper, silicon and silicon carbide.
 6. A lightemitting diode according to claim 5 further comprising a second titaniumlayer between the carrier substrate and said bonding metal structure. 7.A light emitting diode according to claim 6 further comprising a secondplatinum layer between said second titanium layer and said bonding metalstructure.
 8. A light emitting diode according to claim 1 wherein saidbonding metal structure is less than 6 microns thick.
 9. A lightemitting diode according to claim 1 wherein said bonding metal structureis less than 3 microns thick.
 10. A light emitting diode comprising: acarrier structure comprising a first platinum barrier layer and a nickellayer on said first platinum barrier layer; a growth structurecomprising a growth substrate, light emitting epitaxial layers on saidgrowth substrate, and a metal bonding system on said epitaxial layersfor joining to the carrier structure; said metal bonding systempredominating in nickel and tin joining said epitaxial layers to saidcarrier structure with a second platinum barrier layer between saidepitaxial layers and said nickel layer; and wherein the metal bondingsystem is configured so that when said metal bonding system on saidgrowth structure and said nickel layer on said carrier structure arejoined and heated, said respective platinum barrier layers substantiallyprevent tin from migrating through either of said first or secondplatinum barrier layers; wherein said metal bonding system includes goldin an amount greater than 0.3 percent and less than 36 percent by weightof said bonding metal system.
 11. A light emitting diode according toclaim 10 wherein said growth substrate is selected from the groupconsisting of silicon carbide and sapphire, said epitaxial layerscomprise Group III nitrides, and said carrier substrate is selected fromthe group consisting of silicon and silicon carbide.
 12. A lightemitting diode according to claim 11 wherein said epitaxial layerscomprise gallium nitride and said carrier substrate comprises silicon.13. A light emitting diode according to claim 10 further comprising: aflash layer of gold on the nickel-tin portion of said metal bondingsystem; and a flash layer of gold on said nickel layer on said carrierstructure for joining said growth structure and said carrier structureat said respective gold flash layers.
 14. A light emitting diodeaccording to claim 10 wherein the thickness of said tin layer in saidgrowth structure is between about five and 10 times the thickness ofsaid nickel layer in said growth structure.
 15. A light emitting diodeaccording to claim 10, wherein said metal bonding system comprises atitanium adhesion layer, and wherein said carrier structure comprises asecond titanium adhesion layer.